Identification and treatment of an efficiency anomaly in a symmetrically ruled grating illuminated at normal incidence

► We report performance of a grating designed for simultaneous use in ±1 orders. ► This 1200 g/mm grating was intended for use at 121.6 nm under normal incidence. ► Efficiency tests at the desired normal incidence configuration revealed an anomaly. ► The anomaly disappeared after a different MgF2 coating was applied. ► A second, identical grating coated with the 2nd MgF2 recipe also shows no anomaly.